Optical smoke detectors include at least a light source, for example a light-emitting diode operating in the infrared range, and a light-sensitive receiver, for example a photo element. The radiation of the light source and the field of vision of the light-sensitive receiver are collimated. The elements are disposed so that the light-sensitive receiver does not directly receive the radiation from the light source. Smoke detectors of this type rely on the principle that aerosols entering the detector chamber reflect the light radiation more or less. This results in a scattered radiation which is received by the receiver which is triggered and generates an alarm signal provided the scattered light radiation has a predetermined intensity.
The detector chamber of course needs at least an opening through which the smoke may enter the detector chamber. However, an opening in the chamber makes possible the entering of light. One tends to design the optical system in the chamber so that it is shielded against entering light as far as possible. Light entering from the environment is scattered by multiple reflection on the walls of the chamber. The light source disposed in the chamber provides scattered radiation, too. Accordingly, the environmental light and the light source result in a combined scattered radiation which changes in response to the contamination of the detector chamber walls. As there is an opening for the smoke, contamination can not be prevented. Increasing contamination leads to an increased proportion of the scattered radiation. Consequently, the scattered radiation reaches levels which exceed the threshold value of the receiver. Accordingly, there will result false alarm which is most objectionable in fire prevention systems.
The proportion of the radiation from the light source which impinges on the receiver when smoke is in the chamber, is at most 1%. This clearly shows how severely background or noise radiation increased by contamination affects the detecting system. An increase of the background radiation results in an increased sensitivity of the smoke detector. This means that small volumes of smoke which do not yet indicate a danger cause an alarm. Thus, a false alarm may be given at a time in which the noise radiation is not yet sufficient to reach the threshold value of the receiver.